The present invention relates to an image forming and recording apparatus for printing characters and images on a recording medium, such as papers and OHP (Over-Head Projection) sheets or films, etc., and in particular, it relates to a fixing apparatus for fixing unfixed toner held on the recording medium.
As a fixing apparatus for achieving high-speed fixing, such as, being able to take a long time for heating and applying pressure on a recording medium, a pressure belt-type fixing apparatus is already known, wherein the recording medium is in contact with a fixing roller, in particular, on a surface of which the toner adheres, while being in contact with an endless belt, on a surface of which the toner does not adhere, and thereby pressing the recording medium by means of pressing members. In such the pressure belt-type fixing apparatus, it is very important to apply thermal energy and pressing energy appropriately, within a section where the fixing roller and the endless belt are in contact with, through which a paper passes, (hereinafter, being called by a “nip portion”, and the width thereof is called by a “nip width”).
In Japanese Patent Laying-Open No. Hei 8-166734 (1996) <JP-A 08-166734>, there is described a fixing apparatus for an image forming apparatus, comprising: a heat fixing roller integrating a heat generating means therein and being rotationally driven; a pressure belt being wound around the heat fixing roller, so as to be contact therewith; a pressure roller, being provided as one of a plural number of rollers for suspending the pressure belt, for pressing the heat fixing roller at a downstream portion of a pressure portion defined between the heat fixing roller and the pressure belt, in the rotating direction of the heat fixing roller, so that compress deformation is produced in an elastic layer of the heat fixing roller; and an auxiliary pressure roller, being provided in an upstream portion of the pressure portion, and being pressed onto the heat fixing roller through the pressure belt. In such the fixing apparatus, a total of pressure loading between the auxiliary pressure roller and the heat fixing roller, and pressure loading due to tension of the pressure belt, being wound around the heat fixing roller, so as to be contact therewith, are set to be equal to or greater than the pressure loading of the pressure roller. With this, the recording sheet can be transferred at velocity or speed near to the peripheral velocity of the heat fixing roller where no deformation is produced, by bringing the friction power prevail between the heat fixing roller and the recording sheet, in particular, between the pressure portion of the auxiliary pressure roller and the press portion of the pressure roller, i.e., a portion where a small distortion appears on the peripheral surface of the heat fixing roller.
Also, in Japanese Patent Laying-Open No. 2001-228731(2001) <JP-A 2001-228731> is described a fixing apparatus, in which a pressure pad is used as a pressure member, and a porous resin member is provided opposing to the endless belt of the pressure members, for the purpose of lowering the friction, as well as, maintaining a stable traveling performance by preventing driving torque of the belt (i.e., the endless belt) from increasing due to variation with time, and wherein a lubricant is held within that porous resin member. With such the fixing apparatus, for the purpose of supplying the lubricant with stability, a felt is provided as a means for supplying the lubricant with stability, on an inner periphery surface of the endless belt.
In the fixing apparatus shown in the Japanese Patent Laying-Open No. Hei 8-166734, the pressure loading by means of the auxiliary pressure roller, the pressure loading caused due to tension of the pressure belt, and the pressure loading of the pressure roller are determined in amplitude thereof, by paying attention on a relationship between the peripheral velocity of the heat fixing roller and the transfer speed of the recording sheet, but it cannot be said that consideration is fully paid for distribution of operations or functions, such as, preheating of the recording medium at the nip portion, melting and fixing of toner, and strip of the recording sheet from the fixing roller, and/or distribution of the respective pressures at that time. Also, among the respective pressure loadings by means of the auxiliary pressure roll, due to tension of the pressure belt, and by the pressure roller, the pressure loading of the pressure roller is at the maximum.